Fat-suppressed MRA
The signal from fat sometimes obscures vessels on TOF MRA. Can fat suppression be used to improve image quality?
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Since time-of-flight MR images are T1-weighted and reformatted using a maximum intensity projection (MIP) method, high signal from fat may obscure vessels or mimic pathology. This is particularly problematic at the skull base where abundant fatty marrow may create artifacts around the carotid and vertebral arteries. As such, a number of centers have explored methods to suppress fat through chemical shift (CHESS), Dixon, or water excitation (WE) techniques. An example using WE is shown below:
TOF MRA (conventional). Note high signal from fat in marrow, orbits and subcutaneous tissues.
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TOF MRA using water excitation. The high signal is gone and ophthalmic arteries (arrows) are now well seen.
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Some authorities have reported good success with these methods, expecially for visualizing smaller vessels like the ophthalmic and lenticulostriate arteries. Other sites (including my own) have found these methods create new artifacts that may be worse than those from residual fat signal. Presumably these artifacts (including spurious loss of intravascular signal around curves and bends) are due to imperfect slice profiles with unintended saturation effects.
Keep in mind that fat suppression (or water excitation) methods will not suppress "shine-though" of hematomas, CSF-flow phenomena, motion, or gadolinium — all of which may be much more troublesome than fat. While keeping an open mind toward new developments, I therefore do not recommend these methods be used for routine TOF MRA.
Keep in mind that fat suppression (or water excitation) methods will not suppress "shine-though" of hematomas, CSF-flow phenomena, motion, or gadolinium — all of which may be much more troublesome than fat. While keeping an open mind toward new developments, I therefore do not recommend these methods be used for routine TOF MRA.
References
Gizewski ER, Ladd ME, Paul A, et al. Water excitation: a possible pitfall in cerebral time-of-flight angiography. AJNR Am J Neuroradiol 2005; 26:152-155.
Mirowitz SA. Apparent vascular occlusion on cranial TOF MRA with peripheral presaturation technique. J Comput Assist Tomogr 1993;17:927–931.
Gizewski ER, Ladd ME, Paul A, et al. Water excitation: a possible pitfall in cerebral time-of-flight angiography. AJNR Am J Neuroradiol 2005; 26:152-155.
Mirowitz SA. Apparent vascular occlusion on cranial TOF MRA with peripheral presaturation technique. J Comput Assist Tomogr 1993;17:927–931.
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